What is the significance of defining prefixes in units?

The physical quantities have wide range of variation as we come from microscopic to macroscopic scale. A single unit would not be sufficient to measure a physical quantity over a wide range. 
Usually one unit is defined for a physical quantity and the size of the unit is increased or decreased by using prefix with that single unit. 
For example, let us consider the thickness of a paper of the order 0.0001 m. MKS unit of length is metre. Thus, to measure the thickness of one page, we use the unit millimeter or micrometer. Milli and micro are the prefixes that decrease the unit meter by factor 10³ and 10⁶ respectively.